Sensing of gas concentrations is important for various terrestrial and space applications. Some conventional ambient temperature oxygen sensors are optical-based, including expensive and complicated instrumentation. These types of systems can also be susceptible to humidity effects and cross-interference. Other conventional ambient temperature oxygen sensors are based on conventional electrochemical cells, which include aqueous electrolytes. Both types of sensors are not packaging-friendly and the electrochemical cells are susceptible to leaking, particularly in space or variable pressure applications. These types of sensors are also difficult to miniaturize.
A few conventional solid state oxygen sensors are high temperature electrolyte-based. The electrolyte needs to be heated to a high temperature (e.g., 600° C. or more) to achieve practical levels of sensitivity. This is due to the intrinsic properties of the solid electrolyte sensing materials. Such conventional sensors also have considerable power consumption requirements. In an effort to reduce the power consumption of such sensors, NASA Glenn Research Center and Case Western Reserve University have successfully developed a high temperature oxygen microsensor using Yttria-Stabilized Zirconia (“YSZ”) as a solid state electrolyte sensing material. The sensor has the advantages of a wide detection range, small size, easy batch fabrication, relatively low cost, and low power consumption. However, although such a design significantly reduces power consumption due to its micro size, the sensors still need to be heated to a high temperature (e.g., 600° C.) to achieve a practical sensitivity to oxygen. These high temperature sensors are still not suitable for some space applications, such as those in close contact with astronauts, and can limit the battery operation of the sensor system in portable units.
In view of these disadvantages, in oxygen detecting microsensors, use of a Nafion® film may be beneficial. A significant advantage of Nafion®-based sensors is that they can detect oxygen at ambient temperature, which can significantly save power consumption required for heating. In another approach that can decrease power consumption, the system can be operated in a potentiometric mode. As such, no power needs to be applied for the sensor to operate. Rather, only an electrometer, or potentiometric meter, is needed for measuring the voltage changes between a working electrode and a reference electrode under different gas environments. Accordingly, the power consumption is extremely low, making such sensors particularly useful for many applications.
However, in lower humidity environments, the ability of Nafion® to operate effectively as a detecting medium breaks down. For example, in a desert environment, Nafion® film could gradually lose conductivity when its moisture content as a whole drops below 20%. This is because Nation® is more conductive in higher humidity environments and less conductive in lower humidity environments since the presence of sufficient water within the electrolyte material is critical for its effective operation. While Nafion® may work reasonably well for oxygen detection in environments with humidity, such as within a spacesuit or standard ambient humidity environments, Nafion® is suboptimal for less humid environments, such as higher temperature environments with little or no humidity. Accordingly, a low power sensor that operates in environments with varying humidity may be beneficial.